1. Field of the Invention
The present invention relates to an image processing apparatus capable of thinning data output from a solid-state image pickup device having a certain resolution.
2. Related Background Art
In a conventional image input apparatus having a solid-state image-pickup device, an optical low-pass filter is mounted in front of the image pickup device in order to exclude from an image signal produced by the device contain return noises of spatial sampling. An optical low-pass filter band-limits spatial frequencies higher than (1/2)f (mosaic filter) or (1/3)f (pure color vertical stripe filter) where f is a horizontal pixel drive frequency, although this depends upon an arrangement of color filters on the solid-state image pickup device.
In a conventional image input apparatus of the above-described type having an area sensor composed of 500.times.M horizontal pixels and 500.times.n vertical pixels, spatial frequencies higher than (1/2)f (MHz) or (1/3)f (MHz) are band-limited where f is a drive frequency for all pixels in one line. For example, in thinning 500.times.M horizontal pixels and 500.times.n vertical pixels of the area sensor into 500.times.M/M horizontal pixels and 500.times.n/n vertical pixels (where M and n are each an integer equal to at least 3 or 4 or larger), return noise components will be contained in the base band signal.
If such an image input apparatus is applied to a video camera for example, an image thinned into 500 horizontal pixels and 500 vertical pixels and displayed on a general monitor or EVF (electric viewfinder) has return noises with much moire. Therefore, it is difficult to adjust a focus, or the angle of view.
Apart from the above, in order to display pixel signals of the image pickup device on a monitor such as EVF, it is necessary to read all pixel signals synchronously with a television signal frequency. However, as the number of total pixels of the device is increased for high resolution, the drive frequency for driving the device becomes considerably quite high. Furthermore, as the drive frequency becomes high, it becomes difficult to obtain sufficient frequency characteristics of signal processing circuits including such as a sample/hold (S/H) circuit, amplifier, analog/digital converter, and memory, resulting in a complicated circuit arrangement and high cost, even where only poor performance is attained.